(1) No studies directly measured Sars-CoV-2 transmission from reinfected individuals to their contacts. However, there was evidence showing that reinfections are rare. Studies that have followed people for 5 to 7 months after recovery from a Sars-CoV-2 infection have estimated that the protective effect of previous Sars-CoV-2 infection is very high (81%-100%) during that period. It should be noted however, that many of these studies were carried out before the emergence of Sars-CoV-2 variants of concern and there is weak evidence that immunity induced against previously circulating Sars-CoV-2 strains may not have the same potency against variants of concern, in particular B.1.351 and P.1.
(2) Among vaccinated individuals, ECDC’s review showed the following: evidence of the impact of vaccination on the risk of transmission is available from just one study, which suggests that vaccination of a household member reduces the risk of infection in susceptible household members by at least 30%. There is evidence that vaccination significantly reduces symptomatic or asymptomatic infection in vaccinated individuals, although the vaccine efficacy varies by vaccine product and target group. There is also some evidence of lower viral load and shorter duration of shedding in vaccinated individuals as compared to unvaccinated individuals, which could translate into reduced transmission. Many of the studies on vaccine effectiveness were carried out before the emergence of Sars-CoV-2 variants of concern, and there is some evidence that vaccine efficacy could be decreased for some of the variants, in particular for B.1.351 and possibly also P.1.